A so-called on-board electrical system, which is operated as standard at 14 V, is provided in motor vehicles comprising internal combustion engines for the purpose of supplying power to the electric starter for the internal combustion engine and other electrical apparatuses of the motor vehicle (see FIG. 1). During starting of the internal combustion engine (not illustrated), a voltage which is dependent on the state of charge of a starter battery 10 which is provided for supplying power is made available to a starter 11, which starts the internal combustion engine (in the example of FIG. 1: when the switch 12 is closed by a corresponding starter signal), by means of the on-board electrical system. If the internal combustion engine is started, said internal combustion engine drives an electrical generator 13-1 (“alternator”) which then generates a voltage of approximately 14 V and makes said voltage available to the various electrical consumers 14-1 in the motor vehicle by means of the on-board electrical system. In the process, the electrical generator 13-1 also recharges the starter battery 10 which is subject to loading as a result of the starting process.
So-called micro-hybrid motor vehicles can be expected to become widespread in the near future. In contrast to “pure” hybrid motor vehicles, this class of motor vehicles has only an internal combustion engine for driving purposes. Nevertheless, apparatuses for recovering kinetic energy (recuperation), for example during braking, are provided, but the energy recovered in electrical form in the process is not used for driving purposes but, amongst other things, for an automatic start/stop system and other electrical subsystems, in order to have to use the electrical generator which is operated by the internal combustion engine as seldom as possible, this reducing the loading on the internal combustion engine and therefore fuel consumption by said internal combustion engine.
However, micro-hybrid motor vehicles which are already in series production today have massive problems in respect of the service life of the lead-acid batteries which are usually used since they are subject to a considerable additional charge throughput compared to classical motor vehicles. The reason for this is that the electrical consumers have to be supplied with power by the battery during the stop phases in which the internal combustion engine is switched off. Secondly, the battery which is more rapidly discharged in this way is recharged during the relatively short phases of recuperation, this again constituting loading of the battery. This leads to the batteries in micro-hybrid vehicles sometimes not reaching a service life of two years. Frequent replacement of a battery leads to problems in respect of the reliability of the motor vehicles and annoys customers.
A further problem in motor vehicles which are equipped to current standards is that it is problematical to supply power to high-power electrical consumers, for example electrical air-conditioning compressors or electrical heating systems, with an on-board electrical system which is operated at 14 V because, on account of the relatively low voltage, said consumers have to be fed with very high currents in order to be able to provide the required power.